A rolling bearing unit fitted with an encoder as described in, for example, Japanese Unexamined Patent Publication No. TokuKaiSho 64-21219 has heretofore been known, to rotatably support wheels of a vehicle relative to a suspension unit as well as to detect the rotational speed of the wheels in order to control an anti-lock brake system (ABS) or a traction control system (TCS).
Moreover, a structure in which an encoder is fitted in a part of a seal ring, which is used for closing an opening at an end portion of the rolling bearing unit, has heretofore been known, as described in, for example, Japanese Unexamined Patent Publications Nos. TokuKaiHei 5-26233, 5-238369, 6-281018, 9-274051 and 11-44702, and European Patent Publication EP 495 323 A1.
When detection of the rotational speed of the wheels is performed magnetically, encoders in which the magnetic property changes alternately in the circumferential direction (generally, at even intervals) are used. In order that the magnetic property changes alternately in the circumferential direction in such encoders, a plurality of removed portions and solid portions are alternately formed in the circumferential direction on a magnetic material, or a permanent magnet in which S poles and N poles are arranged alternately is used.
Among these, the encoder made of a permanent magnet is recently increasing in use from the point that the structure on the sensor side is easily made up, and that is maintains detection accuracy at low speeds.
In the case of the combination seal ring fitted with an encoder that is heretofore known, for example, from the description in the above mentioned publications, it has been difficult to incorporate the seal ring into a small rolling bearing unit in a condition where this does not protrude from the end face of a race of the rolling bearing unit, while ensuring sufficient sealing property, and also ensuring a sufficiently large output, regardless of slight fitting errors.